In North America, asphalt-based roofing materials are a popular medium for covering roofs on homes and other structures. Asphalt-based roofing materials may come in shingle or roll form, the shingle being the more widely used material. A typical asphalt shingle has an asphalt substrate and a multitude of roofing granules placed thereon. The roofing granules serve an aesthetic function by providing color to the shingle and thus to the roof.
Colored roofing granules have been prepared in the following manner. First, mineral rock granules of about 420-1680 micrometers (40 to 12 US mesh) are preheated to around 100.degree.-1000.degree. F. (38.degree.-538.degree. C.). A paint slurry containing a pigment is then applied to the heated granules in a mixer. The color coated granules are then further heated in a kiln to about 350.degree. to 1200.degree. F. (175.degree.-650.degree. C.), are cooled, and are subsequently passed to a post-treatment stage where the colored granules are treated with an oil formulation in a rotary mixer. The oil formulation is applied to reduce dust and promote adhesion of the granules to the asphalt substrate. After the oil treatment, the granules are removed from the post-treatment stage, transported, and subsequently applied to the asphalt substrate.
When the granules are secured to an asphalt substrate, it has been found that there is a tendency for oils in the asphalt substrate to creep onto or be adsorbed on the granules' surfaces. This creeping or adsorption of the asphaltic oils on the roofing granules' surfaces causes a discoloration (staining) of the roofing granules and hence the shingles. Although the discoloration is normally removed by weathering after the shingles are placed on a roof, there is nonetheless a disparity in color between what the consumer sees at the time of purchase and what the ultimate color of roof becomes after weathering. This color disparity is especially emphasized when the shingles have a lighter color, particularly white.
The present invention is directed to overcoming the problem of having asphaltic oils adsorb onto and stain the surface of roofing granules. This and other objects and advantages of the invention will be more fully understood in the following description and accompanying figures. It is to be expressly understood, however, that the description and figures are for purposes of illustrating the invention and should not be read in a manner that would unduly limit the definition of this invention.